textproduct: Rapid City

KEY MESSAGES

- Generally warm and dry conditions this week, with the hottest temperatures reaching the 90s Thursday and Friday.

- Elevated to near-critical fire weather conditions to result, with focus especially on Thursday, Thursday night, and Friday.

- Next organized storm system expected this weekend into Monday, potentially bringing much cooler temperatures.

DISCUSSION

(This Evening Through Tuesday) Issued at 213 PM MDT Tue Sep 8 2026

Our main weather concern this week will remain warm to hot, dry, and at times breezy conditions resulting in elevated to near- critical fire weather. On the synoptic scale...a somewhat "undulating" zonal flow will dominate today through Friday, with embedded shortwave troughs brushing across the region this evening and again Wednesday evening. This evening's feature will be the stronger of the two, supporting already-observed 30-40 MPH wind gusts up across the northwestern South Dakota plains. Enough lift may be present with the trough/weak frontal feature to induce development of a few showers during the first half of the overnight tonight roughly north of a line from Gillette to Faith (as reflected with actually decent consensus across nearly all current CAM guidance), despite the best precip potential focusing well to our north over Montana and North Dakota. This scenario was not well captured by the NBM, so blended in some high-res guidance and broadbrushed light 10-30% PoPs tonight up across this region to cover any development. Instability seems limited, and have left thunder out of the forecast for now. Regardless, significant impacts are not expected from this activity with very little QPF. Wednesday evening's feature is actually sharper/stronger in the upper levels of the atmosphere, but weak at the surface, and is currently expected to cross the region dry with little to no impact. The hottest temperatures of the week are expected Thursday and Friday, returning to the 90s.

Friday, long-range deterministic models continue to key in on two primary features. The first will be a large low pressure center crossing Canada to our north, which will assist in tightening the pressure gradient and increasing northwesterly winds Friday afternoon and evening (gusts 25-40 MPH across the region). This will especially be a fire weather concern...not expecting Wind Advisory-level winds at this time. Some hint of troughing extending southwestward from this parent low may also help support some shower/thunderstorm potential starting late Friday afternoon into the first part of Friday night, although confidence in this period is low. Meaningful or widespread QPF seems unlikely. The second main feature will be a second closed low rotating down the back side of this Canadian parent low and then swinging eastward/inland across the northern Rockies. Moisture advecting in on southwest flow ahead of this low, along with increasing upper- level support, will bring us a better chance of more organized precipitation sometime in the Saturday/Sunday/Monday timeframe, although amounts are still very much in question at this range. (WPC guidance currently supports about one-quarter inch, with potentially higher amounts over the south-central South Dakota plains.) This system will moisten up our relative humidity values taking the edge off fire weather concerns, and COULD spell at least a temporary end to the heat! NBM high temperatures currently top out at 70 to 80 degrees Saturday, and 65 to 75 degrees Sunday and Monday, with overnight lows in the 40s through the period. The overall magnitude of the cooler air during this period has perhaps moderated a bit in the guidance over the past 24 hours, although Saturday itself has trended a bit cooler. Keep in mind that our actual eventual temperatures (and QPF amounts) will be tied to the strength and track of this low. Looking at 500mb height cluster analysis...some differences between clusters are noted Saturday in the position of the incoming low/trough axis (as far east as Idaho but as far west as off the Oregon coast), with EOF patterns confirming the majority of model uncertainty is currently tied to speed/position of the low. Most clusters currently remain dry or modest with respect to QPF. Similar overall observations by Sunday, but the stakes will be higher at that point with the low in much closer proximity to Wyoming and South Dakota, meaning significant differences in both potential temperatures and QPF. 2 clusters (about 65% of the ensemble space) support over 0.25" of QPF and cooler temperatures tied to a faster low, while another 2 (about 35% of members) support much drier conditions tied to a further west low with significantly warmer temperatures remaining in place. (Thus, there is a small chance...currently about 35 percent...that our pattern change could be delayed and Saturday's forecast could trend toward another warmer and continued elevated fire weather day.) By Sunday night and Monday, confidence increases much more comfortably in cooler temperatures and showers over the region. No deviations were made from NBM and WPC guidance in the forecast from Friday onward at this time.

AVIATION

(For the 18Z TAFS Through 18Z Wednesday) Issued At 1107 AM MDT Tue Sep 8 2026

VFR conditions are expected to prevail through the valid TAF period. Light, and at times, variable winds this afternoon will turn breezy at the KRAP terminal by 06z. Winds will be out of the northwest at 10 knots gusting up to 20 knots.

FIRE WEATHER

Issued At 213 PM MDT Tue Sep 8 2026

Warm to hot, dry, and episodically breezy conditions remain expected this week, continuing to support widespread elevated to near-critical fire weather conditions through Friday. A low pressure system approaching the region this weekend then looks to potentially support cooler temperatures and higher humidity, although rainfall is more uncertain.

Today and Wednesday...afternoon minimum relative humidity values will generally range from 10-25 percent...driest across northeast Wyoming...but overlap with critical winds seems limited and localized. Enhanced wind gusts of 30-40 MPH have developed as forecast and remain expected through this evening across the northwestern South Dakota plains as a shortwave trough brushes us from the north (resulting in very high grassland fire danger), with lower gusts of generally 15-25 MPH Wednesday across the region. The aforementioned trough may also be able to support a few stray showers early tonight north of a line from roughly Gillette to Faith, but the potential for wetting rains/meaningful QPF is near zero with any activity that develops. Thus, the weather supports fuels remaining receptive to fire, perhaps drying more with time.

Thursday and Friday...things turn a bit more interesting and potentially more concerning, featuring the hottest and driest conditions of the week. RH values will plummet to 6-18 percent Thursday and 10-20 percent Friday across most of the forecast area with potentially very poor overnight recovery Thursday night into Friday morning (only 20-30 percent from the Black Hills and Fall River County area westward across northeastern Wyoming). And, while winds may be modest Thursday, they will increase Friday afternoon and evening as the pressure gradient tightens south of and ahead of two large low pressure systems, currently forecast to gust 25-40 MPH regionwide per NBM guidance. Thus, this Thursday, Thursday night, and Friday period may be the one to watch for our next potential Red Flag Warning opportunity.

UNR WATCHES/WARNINGS/ADVISORIES

SD...None. WY...None.